With the recent demand for saving energy, car tires are required to have a small rolling resistance and a low fuel consumption.
A part of the motive energy transmitted from a car engine to the drive wheels does not act effectively, but is absorbed owing to the heat build-up of the tire, the friction between the road and the tire, etc. The energy loss due to the freely rotating wheels varies largely depending upon the construction of the tire, carcass material, etc., but is roughly estimated in terms of percentage under the following itemized causes:
Friction loss on the road surface--about 10% PA1 Hysteresis loss of tread rubber--50-60% PA1 Loss due to the other tire construction materials--30-40%
Seeing that the energy loss due to the hysteresis loss of a tread rubber is the largest, a decrease in the hysteresis loss might result in a tire whose energy saving effect is large and whose rolling resistance is low.
On the other hand, braking performance of a tire has a correlation with hysteresis loss of the tread rubber. In general, a rubber of low hysteresis loss has a poor braking performance, so that a low energy consumption and a good braking performance of a tire are not compatible with each other.
Since a rubber of low hysteresis loss has a good resilience, the surface of the tread comprised of such rubber, upon coming into contact with a road, conforms to the road surface texture, but is restored to its initial shape immediately when out of engagement from the road surface and becomes smooth shortly before the wheel is rotated to once again come into contact with the road. To the contrary, a rubber of high hysteresis loss has a poor resilience, so that the tread comprised of this rubber, when in contact with the road, sets its surface into a rugged pattern, which remains unrestored while the wheel is rotated one revolution. As a result, the tread surface thus deformed again comes into contact with the road in the non-smooth state, so that there occurs a great friction between the tire and the road surface, which leads to a good braking performance.
For this reason, the lowering of rolling resistance and the improvement of braking performance are, in general, not compatible with each other and are in antinomy relationship. Be that as it may, to lessen energy consumption of a tire at the sacrifice of braking performance cannot be accepted from the viewpoint of safety.
Accordingly, an essential object of this invention is to provide a pneumatic tire having both a low rolling resistance and a good braking performance.
With a view toward achieving the foregoing object, the present inventors have investigated the relationship between rolling resistance and braking performance of a tire and have found out that rolling resistance is related to the hysteresis loss of a rubber constituting the entire tread portion whereas braking performance depends largely upon the properties of a rubber constituting the surfaces of both the tread side zones where contact pressure of them upon the road is high. Hence it has turned out that the antinomy as described above can be solved substantially by constructing a tire so that both tread side zones are each comprised of a rubber having a high hysteresis loss and accordingly, a good braking performance, and the center portion of the tread is comprised of a rubber having a low hysteresis loss, whereby there is obtained a tire having a small rolling resistance and a good braking performance, from which finding the present invention has been accomplished.